This application claims Paris Convention priority of Japanese Application No. 2001-342002 filed Nov. 7, 2001, the complete disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a crystal-pulling apparatus for pulling and growing a monocrystalline silicon ingot according to the Czochralski method (CZ method hereinafter), and a method therefor.
2. Description of the Related Art
The CZ method has been employed as one of the methods for growing silicon single crystals. The CZ method proceeds as follows. Firstly, a polycrystalline silicon material is heated in a quartz crucible to a temperature higher than the melting point of that silicon material, to give a silicon melt. Then, a seed crystal is dipped into the silicon melt kept in the quartz crucible. After the seed crystal has been dipped and itself partially melted, pulling of the seed is started. During pulling, first a thin crystal section, called a neck, is grown to eliminate dislocation growth into the ingot. Then a crystalline ingot is developed from the neck as the crystal is allowed to increase its diameter gradually, so that first a cone-shaped portion, next a shoulder portion, and then a cylindrical portion with a uniform diameter may be formed. The crystalline seed, after having undergone the above steps, grows into a monocrystalline silicon ingot in the form of a solid cylinder.
While a silicon crystal grows according to the CZ method, the internal wall of a quartz crucible is in contact with the silicon melt and will gradually dissolve as a result of the reaction as represented by the following formula (1).
SiO2+Sixe2x86x922SiOxe2x80x83xe2x80x83(1) 
As shown (FIG. 12), the majority of SiO dissolving into the silicon melt (1) will evaporate, as SiO gas, from the free surface 1a of the silicon melt (1), whereas a small amount thereof will be incorporated through the solid-liquid interface (1b) into the monocrystalline silicon ingot (3), to supply the ingot with oxygen which is an impurity. During the early phase of pulling, the contact area between the silicon melt (1) and the internal wall of the quartz crucible (2) is comparatively large, and thus the concentration of oxygen dissolving into the silicon melt (1) becomes very high. However, when the crystal growth process advances and a monocrystalline silicon ingot (3) starts to form as shown (FIG. 13), the surface (1a) of the silicon melt (1) gradually drops, and the contact area between the silicon melt (1) and the internal wall of the quartz crucible (2) becomes smaller compared with what is observed during the early phase of pulling. The reduction of the contact area between the crucible and the melt will cause the concentration of the oxygen transferring from the crucible (2) into the silicon melt (1) to decrease. Accordingly, a fully grown monocrystalline silicon ingot (3) will show an uneven oxygen distribution along its longitudinal axis: more specifically, the oxygen concentration will vary depending on whether the oxygen measurement is made at the seed end of the crystal, in the mid portion of the crystal, or at the tail end of the crystal.
N-type monocrystalline silicon ingot which is heavily doped with an element such as As, P or Sb is favored as a starting material for epitaxial wafers (epi-wafers hereinafter) used in the power discrete market, and its production tends to increase. Two of the essential requirements for a good epi-wafer substrate are its bulk resistivity and its intrinsic gettering capability. The intrinsic gettering capability of a wafer is closely related to the intrinsic oxygen concentration that has been grown into the crystalline structure. It was found in the past that there is a direct correlation between the bulk resistivity of a N-type silicon crystal and the oxygen concentration of such a heavily doped N-type monocrystalline silicon crystal. The lower the resistivity of a specific crystal is, the lower is the oxygen concentration that a crystal incorporates naturally into the crystal structure. The reason for this behavior is partially that elements such as As and Sb evaporate easily from the free surface of the melt while forming oxygen compounds, therefor reducing the oxygen amount in the melt. Take antimony (Sb) as an illustration. If Sb is in a silicon melt, it will bind with oxygen transferred to the melt, to encourage the further dissolution of oxygen, and then it will evaporate alone or as a compound in the form of Sb2O. As a result, if a silicon melt contains a high concentration of Sb, the concentration of coexistent oxygen will decrease, and thus it is difficult to achieve high oxygen incorporation into the a heavily doped Sb-type crystal. As seen above, it has been difficult to obtain an N-type monocrystalline silicon ingot that presents a low resistivity and a high oxygen concentration simultaneously.
An object of this invention is to provide a crystal-pulling apparatus for pulling and growing a monocrystalline silicon ingot whereby it is possible to produce a heavily doped N-type monocrystalline silicon while adjusting the oxygen concentration of the ingot to a desired level, and a method therefor.
A further object of this invention is to provide a crystal-pulling apparatus for pulling and growing a heavily doped N-type monocrystalline silicon ingot whereby it is possible to produce a monocrystalline silicon ingot whose oxygen concentration at the seed end is kept at a level determined by natural equilibrium conditions, while the reduction in oxygen concentration at the cylindrical portion and at the tail end which would be otherwise introduced, is effectively restrained, and a method therefor.
A first aspect of this invention relates to an improved crystal-pulling apparatus for pulling and growing a monocrystalline silicon ingot comprising, within a chamber, of a quartz crucible containing a silicon melt from which a monocrystalline silicon ingot will be pulled; a graphite crucible container which surrounds the outer wall of the crucible at the external base surface of the quartz crucible to support it; and a heater which surrounds the outer wall of the crucible container and is intended to heat the silicon melt. Its constitution has a further notable feature: a spacer which has a top area smaller than the area of the external base surface of the quartz crucible and consists of a material having a higher melting point than that of silicon, is inserted between the external base surface of the quartz crucible and the internal base surface of the crucible container while a seed crystal is pulled to produce a monocrystalline silicon ingot.
According to the first aspect of this invention, a specific property characteristic of the quartz crucible, that is, softening of a quartz during the pulling of a monocrystalline silicon ingot is utilized. When a quartz crucible softens, being heated by a heater to the softening temperature, the base of the quartz crucible will gradually undergo deformation, so that because of its own weight and the weight of the silicon melt kept in the crucible, the a gap formed between the crucible and the crucible container, will be abolished. If the quartz crucible starts to deform, its base will gradually become convex upwards, and thus the internal base area of the crucible will increase. The increased internal base area of the crucible will cause the area of the internal base of the quartz crucible in contact with the silicon melt to increase. Further, because then the silicon melt is raised upwards relative to a given level on the internal wall of crucible, provided that other things remain invariable, the surface of the silicon melt relative to the internal side-wall of crucible would be also raised. This implicit elevation of the surface of the silicon melt will retard the downfall of the same surface which will be brought about in association with the pulling of a monocrystalline silicon ingot. Thus, the downfall of the surface of the silicon melt observed during the formation of a silicon crystal ingot, particularly during the formation of the cylindrical part of the ingot will proceed at a slower pace than would be otherwise observed. Additionally to the increase in the melt-crucible interface a variation in the flow pattern of the melt will occur, due to the change of the crucible bottom shape.
A second aspect of this invention relates to a crystal-pulling apparatus for pulling and growing a monocrystalline silicon ingot comprising a plurality of openings formed on the base of the crucible container symmetrically around the center of the same base, the plurality of spacers fitted to the openings, and a spacer disposing means for raising and lowering the spacers, wherein the plurality of spacers can be raised to a same height above the openings or sunk to a same level in the openings by the spacer disposing means.
According to the second aspect of this invention, if the apparatus is so operated as to cause the base of the quartz crucible to be raised according to the pulling length of a monocrystalline silicon ingot, the base of the quartz crucible will be deformed to have a more enhanced upward convexity than would be otherwise observed. Through this deformation, the surface area of the internal base of the quartz crucible will increase, and thus the area of the internal base of the quartz crucible in contact with the silicon melt will increase. Further, the surface of the silicon melt would be raised through this deformation provided other things remain invariable. Therefore, it will be possible to control the sinking speed of the surface of the silicon melt which will be inevitably brought about in association with the pulling of a monocrystalline silicon ingot, by introducing and adjusting this implicit elevation of the surface of the silicon melt according to the pulling length of a monocrystalline silicon ingot.
Additionally to the increase in the melt-crucible interface a variation in the flow pattern of the melt will occur, due to the change of the crucible bottom shape. The shape change can be controlled by introducing and adjusting the implicit elevation of the surface of the internal base and by selecting various different shapes and numbers for the plurality of spacers.
In summing up, according to this invention, it is possible:
to increase the surface area of internal base of a quartz crucible by emphasizing the upward convexity of that base, thereby enlarging the area of the same base in contact with a silicon melt,
to elicit, if other things remain invariable, an elevation of the surface of a silicon melt which will increase the area of the internal wall of the quartz crucible in contact with the silicon melt, this implicit elevation of the surface of the silicon melt diminishing the speed at which the contact area in question is reduced, which is brought about in association with the pulling of a monocrystalline silicon ingot,
to change the flow pattern in the silicon melt by elevating and changing the internal wall shape at the bottom of the crucible in a way that the flow of high oxygen melt to the crystal melt interface is increased, and
to increase the oxygen dissolution rate from certain areas of the crucible wall by deforming crucible areas to form convex crucible wall areas.
As a consequence, it is possible to control the reduction in concentration of oxygen in a silicon melt from which an N-type silicon monocyrystalline ingot heavily doped with impurities and having a low resistivity is prepared, or in other words, to maintain, at a desired level, the concentration of oxygen in a silicon melt from which such an N-type silicon monocyrystalline ingot is prepared. Because this invention is based on a specific property characteristic with a quartz crucible which will soften while a monocrystalline silicon ingot is pulled, the seed terminal of the silicon ingot will retain the concentration of oxygen as determined by natural equilibrium conditions, while the full grown cylindrical portion and tail end of the same ingot will be relieved of the sharp decline in oxygen concentration which would result without the introduction of this invention.